It is important to note that the animal shown in this series of photos was taken from a trap. Animals in the wild do not split up the back like this, rather the cephelothorax and tail seperate on the dorsal aspect and the animal backs out through the resulting opening. Why animals in traps shed in the manner depicted here is a mystery. One theory is that the lack of natural substrate deprives them of edges to use to leverage the shell off.

Another, more interesting theory is that the animal tries to postpone molting in the trap because molting would leave it vulnerable to attack by other inhabitants in the trap. This may sound far fetched until you learn about the role of environment and chemical signals involved in molting and mating (two activities that are usually linked) and why being stuck in a trap is probably not percieved by the animal as the right place to conduct either of these activities.

At any rate, eventually the proccess can be postponed no longer and the shell splits, due either to changes in fluid concentrations in the tissues, or perhaps to a growth spurt that splits the shell like a pair of jeans that have become too tight. No one seems to know for sure.